In long known practices, positional information respecting object surfaces as been obtained through stereoscopic examination. In such practices, typified by Koper U.S. Pat. No. 3,783,294, the surface is viewed through spaced lenses and photographs are made of the separate views. Characteristics, such as the slope of the surface, may be derived geometrically from the photographic renditions of the surface, for example, as in the Koper patent, by triangulation involving the ratio (X anamorph) of the lengths of the surface evidenced in the photographs.